feat(subtitles): add regex subtitle filters with built-in presets (#318)

- Add `subtitleFilter` config with four presets: parentheses, brackets, speaker prefix and music symbols. Custom regex rules run after the presets.
- Filter primary text where it enters the app (live `sub-text` and parsed cues), so the overlay, sidebar, mining and stats all get the same filtered text.
- Changes hot-reload: the current line is read again and the active track is parsed again, using the filter config as the prefetch parse key.
- Add a custom rule editor in Settings under Behavior → Subtitle Filters that checks patterns as you type.
- Update the config example, the configuration docs and the architecture domains doc, and add a changelog fragment.

config.example.jsonc

@@ -854,6 +854,22 @@
     } // CSS declaration object applied to the subtitle sidebar. Includes color, background-color, and all font properties.
   }, // Parsed-subtitle sidebar cue list styling, behavior, and toggle key.
 
+  // ==========================================
+  // Subtitle Filters
+  // Remove unwanted text from primary subtitles, such as speaker names, readings and sound descriptions.
+  // Applies to the overlay, sidebar, mined sentences and stats.
+  // Hot-reload: filter changes apply to the current subtitle file immediately.
+  // ==========================================
+  "subtitleFilter": {
+    "presets": {
+      "parentheses": false, // Remove text in （） and (), such as speaker names, readings and notes. A line wrapped entirely in parentheses, usually inner thoughts, is kept. Values: true | false
+      "brackets": false, // Remove text in ［］, [], 【】 and 〔〕, such as sound effects and music cues. Values: true | false
+      "speakerPrefix": false, // Remove a short name followed by a colon at the start of a line, as in 山田：元気？ Values: true | false
+      "musicSymbols": false // Remove ♪, ♫ and ♬. A line left with only symbols such as ～ is removed. Values: true | false
+    }, // Built-in filters. Each removes its matches and is off by default.
+    "customRules": [] // Your own regex rules, applied in order after the presets. Each rule is { name, pattern, replacement, enabled }; patterns use the gmu flags, so ^ and $ match per line. Leave replacement empty to remove the match.
+  }, // Remove unwanted text from primary subtitles, such as speaker names, readings and sound descriptions.
+
   // ==========================================
   // Shared AI Provider
   // Canonical OpenAI-compatible provider transport settings shared by Anki and YouTube subtitle fixing.