topics: Add topic list page

The topic-list page app bar should match the Figma design except for its
height.  That part needs to be coordinated with other pages.

Also on the topic-list page app bar, the icon "chveron_down.svg"
(originally "chveron-down-16.svg") comes from:
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=6820-36184&m=dev

and not the "Icons" page, since the "chveron-down" there
appears to be different from the one on the topic list page design.

We also leave out the new topic button and topic filtering.  Those are
out-of-scope for #1158.

---

On the message-list page, "TOPICS" is not aligned to the middle part
of the app bar, that's because we haven't got to #1039 yet.

---

The topic-list implementation is quite similar to parts of inbox page
and message-list page.  Therefore, we structure the code to make them
look similar to compare for changes side-by-side to help with reviewing
what has changed.

Figma design:
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=6819-35869&m=dev

Author: PIG208

assets/icons/ZulipIcons.ttf

@@ -713,6 +713,10 @@
   "@mainMenuMyProfile": {
     "description": "Label for main-menu button leading to the user's own profile."
   },
+  "topicsButtonLabel": "TOPICS",
+  "@topicsButtonLabel": {
+    "description": "Label for message list button leading to topic-list page. (Use ALL CAPS for cased alphabets: Latin, Greek, Cyrillic, etc.)"
+  },
   "channelFeedButtonTooltip": "Channel feed",
   "@channelFeedButtonTooltip": {
     "description": "Tooltip for button to navigate to a given channel's feed"

assets/icons/chevron_down.svg

@@ -1064,6 +1064,12 @@ abstract class ZulipLocalizations {
   /// **'My profile'**
   String get mainMenuMyProfile;
 
+  /// Label for message list button leading to topic-list page. (Use ALL CAPS for cased alphabets: Latin, Greek, Cyrillic, etc.)
+  ///
+  /// In en, this message translates to:
+  /// **'TOPICS'**
+  String get topicsButtonLabel;
+
   /// Tooltip for button to navigate to a given channel's feed
   ///
   /// In en, this message translates to:

assets/l10n/app_en.arb

@@ -584,6 +584,9 @@ class ZulipLocalizationsAr extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'My profile';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Channel feed';
 

lib/generated/l10n/zulip_localizations.dart

@@ -584,6 +584,9 @@ class ZulipLocalizationsEn extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'My profile';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Channel feed';
 

lib/generated/l10n/zulip_localizations_ar.dart

@@ -584,6 +584,9 @@ class ZulipLocalizationsJa extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'My profile';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Channel feed';
 

lib/generated/l10n/zulip_localizations_en.dart

@@ -584,6 +584,9 @@ class ZulipLocalizationsNb extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'My profile';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Channel feed';
 

lib/generated/l10n/zulip_localizations_ja.dart

@@ -593,6 +593,9 @@ class ZulipLocalizationsPl extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'Mój profil';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Strumień kanału';
 

lib/generated/l10n/zulip_localizations_nb.dart

@@ -597,6 +597,9 @@ class ZulipLocalizationsRu extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'Мой профиль';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Лента канала';