viewer: migrate to vue 3, general refactoring and cleanup

Non-exhaustive list of fixes and improvements done at the same time:

- html default background to grey (avoids white flash during init)
- unified links behavior
- added more theme variables
- removed the flex-expand transition (it wasn't working) and replaced it
  with a slide
- fixed LdLoading not centered on the content
- title on removable tags
- fixed an issue with encoded URI from vue-router
- unified Item resource URLs
- removed the iframe for PlainTextViewer (it wasn't working properly)
  and replaced it with a pre
- fixed clear and search buttons tabindex
- fixed the information panel bumping up during the fade animation of
  tag's dropdown
- fixed some focus outlines not appearing correctly
- moved CSS variables to the :root context
- Code cleaning

GitHub: closes #217
GitHub: closes #300
GitHub: closes #297
GitHub: closes #105
GitHub: closes #267
GitHub: closes #275
GitHub: closes #228
GitHub: closes #215
GitHub: closes #112

8 files changed, 85 insertions, 140 deletions

diff --git a/viewer/src/@types/gallery.d.ts b/viewer/src/@types/gallery.ts
index 0b4cfc4..8c0f177 100644
--- a/viewer/src/@types/gallery.d.ts
+++ b/viewer/src/@types/gallery.ts
@@ -1,7 +1,7 @@
 /* ldgallery - A static generator which turns a collection of tagged
 --             pictures into a searchable web gallery.
 --
--- Copyright (C) 2019-2020  Guillaume FOUET
+-- Copyright (C) 2019-2022  Guillaume FOUET
 --
 -- This program is free software: you can redistribute it and/or modify
 -- it under the terms of the GNU Affero General Public License as
@@ -17,10 +17,10 @@
 -- along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */
 
-import { ItemType } from "./ItemType";
-import { SplashScreenConfig } from "./splashscreen";
+import { ItemType } from './itemType';
+import { SplashScreenConfig } from './splashscreen';
 
-export type ItemSortStr = "title_asc" | "date_asc" | "date_desc";
+export type ItemSortStr = 'title_asc' | 'date_asc' | 'date_desc';
 
 export interface Config {
   galleryRoot: string;
@@ -30,39 +30,54 @@ export interface Config {
   splashScreen?: SplashScreenConfig;
 }
 
-export interface Properties {
-  galleryTitle: string;
-  tagCategories: RawTag[];
+// ---
+
+export interface Resolution {
+  width: number;
+  height: number;
 }
-export interface Index {
-  properties: Properties;
-  tree: DirectoryItem;
+export interface Thumbnail {
+  resource: string;
+  resolution: Resolution;
 }
+export type RawTag = string;
 
-export interface OtherItem extends Item {
-  properties: OtherProperties;
+// ---
+
+export interface Downloadable {
+  resource: string;
+  type: ItemType; // unknown
 }
-export interface PictureItem extends Item {
-  properties: PictureProperties;
+export interface OtherProperties extends Downloadable {
+  type: ItemType.OTHER;
 }
-export interface PlainTextItem extends Item {
-  properties: PlainTextProperties;
+export interface PictureProperties extends Downloadable {
+  type: ItemType.PICTURE;
+  resolution: Resolution;
 }
-export interface MarkdownItem extends Item {
-  properties: MarkdownProperties;
+export interface PlainTextProperties extends Downloadable {
+  type: ItemType.PLAINTEXT;
 }
-export interface PDFItem extends Item {
-  properties: PDFProperties;
+export interface MarkdownProperties extends Downloadable {
+  type: ItemType.MARKDOWN;
 }
-export interface VideoItem extends Item {
-  properties: VideoProperties;
+export interface PDFProperties extends Downloadable {
+  type: ItemType.PDF;
 }
-export interface AudioItem extends Item {
-  properties: AudioProperties;
+export interface VideoProperties extends Downloadable {
+  type: ItemType.VIDEO;
 }
-export interface DirectoryItem extends Item {
-  properties: DirectoryProperties;
+export interface AudioProperties extends Downloadable {
+  type: ItemType.AUDIO;
 }
+export interface DirectoryProperties {
+  type: ItemType.DIRECTORY;
+  // eslint-disable-next-line no-use-before-define
+  items: Item[];
+}
+
+// ---
+
 export interface Item {
   title: string;
   datetime: string;
@@ -71,55 +86,44 @@ export interface Item {
   path: string;
   thumbnail?: Thumbnail;
   properties:
-    | OtherProperties
-    | PictureProperties
-    | PlainTextProperties
-    | MarkdownProperties
-    | PDFProperties
-    | VideoProperties
-    | AudioProperties
+    | Downloadable
     | DirectoryProperties;
 }
-export interface Resolution {
-  width: number;
-  height: number;
+export interface DownloadableItem extends Item { // Special unknown item type
+  properties: Downloadable;
 }
-export interface OtherProperties {
-  type: ItemType.OTHER;
-  resource: string;
+export interface OtherItem extends Item {
+  properties: OtherProperties;
 }
-export interface PictureProperties {
-  type: ItemType.PICTURE;
-  resource: string;
-  resolution: Resolution;
+export interface PictureItem extends Item {
+  properties: PictureProperties;
 }
-export interface PlainTextProperties {
-  type: ItemType.PLAINTEXT;
-  resource: string;
+export interface PlainTextItem extends Item {
+  properties: PlainTextProperties;
 }
-export interface MarkdownProperties {
-  type: ItemType.MARKDOWN;
-  resource: string;
+export interface MarkdownItem extends Item {
+  properties: MarkdownProperties;
 }
-export interface PDFProperties {
-  type: ItemType.PDF;
-  resource: string;
+export interface PDFItem extends Item {
+  properties: PDFProperties;
 }
-export interface VideoProperties {
-  type: ItemType.VIDEO;
-  resource: string;
+export interface VideoItem extends Item {
+  properties: VideoProperties;
 }
-export interface AudioProperties {
-  type: ItemType.AUDIO;
-  resource: string;
+export interface AudioItem extends Item {
+  properties: AudioProperties;
 }
-export interface DirectoryProperties {
-  type: ItemType.DIRECTORY;
-  items: Item[];
+export interface DirectoryItem extends Item {
+  properties: DirectoryProperties;
 }
 
-export interface Thumbnail {
-  resource: string;
-  resolution: Resolution;
+// ---
+
+export interface IndexProperties {
+  galleryTitle: string;
+  tagCategories: RawTag[];
+}
+export interface Index {
+  properties: IndexProperties;
+  tree: DirectoryItem;
 }
-export type RawTag = string;
diff --git a/viewer/src/@types/ItemType.ts b/viewer/src/@types/itemType.ts
index 5ef38d8..ecab05c 100644
--- a/viewer/src/@types/ItemType.ts
+++ b/viewer/src/@types/itemType.ts
@@ -18,12 +18,12 @@
 */
 
 export enum ItemType {
-  OTHER = "other",
-  PICTURE = "picture",
-  PLAINTEXT = "plaintext",
-  MARKDOWN = "markdown",
-  PDF = "pdf",
-  VIDEO = "video",
-  AUDIO = "audio",
-  DIRECTORY = "directory",
+  OTHER = 'other',
+  PICTURE = 'picture',
+  PLAINTEXT = 'plaintext',
+  MARKDOWN = 'markdown',
+  PDF = 'pdf',
+  VIDEO = 'video',
+  AUDIO = 'audio',
+  DIRECTORY = 'directory',