feat(app): add workspace file browser to session files screen

[seibe]

Summary

Extends the existing session Files screen from a git-changes-only view into a dual-mode screen with "Changes" and "Browse" tabs. Users can now navigate the full workspace directory tree, not just view staged/unstaged git changes.

Also fixes UTF-8 handling for both file paths and file content, which caused crashes and garbled text with non-ASCII filenames and multi-byte content (CJK, Cyrillic, emoji).

Related issues

• File browser #361 — file browser feature request. The existing Files screen shows git changes and supports file search, but has no directory browsing capability. This PR adds interactive directory navigation.
• Chinese characters display as garbled text when viewing Git filechanges on Android app #199 — Chinese characters display as garbled text when viewing git file changes. Root cause: atob() only supports Latin-1 and corrupts multi-byte UTF-8 sequences. This PR replaces atob() with TextDecoder('utf-8') for file content and adds URL-safe UTF-8 base64 encoding for file paths, fixing the exact issue reported.

What's new

Browse mode

• Directory-based file browsing with path history (back navigation)
• Auto-activates for non-git workspaces (replaces the previous "not under git" error screen)
• File size display, binary file detection by extension (~35 known binary types)
• Hidden files (. prefixed) filtered out
• Mode toggle ("Changes" / "Browse") shown in git repos; non-git defaults to Browse
• Directory listing refreshes on screen focus

UTF-8 path encoding (fixes #199 partially)

• Replaces btoa()/atob() with URL-safe UTF-8 base64 encoding/decoding (utf8ToBase64 / base64ToUtf8 in stringUtils.ts)
• Legacy atob() fallback for paths encoded before this change

UTF-8 content decoding (fixes #199)

• File content now decoded via TextDecoder('utf-8', { fatal: true }) instead of raw atob() binary string
• Invalid UTF-8 sequences are reclassified as binary (prevents mojibake)
• Reduced memory allocation in binary detection (loop vs split().filter())
• Proper empty file handling

Changes by file

• files.tsx: Add Browse mode with directory navigation, mode toggle UI, path bar, auto-switch for non-git workspaces
• file.tsx: UTF-8 safe path decoding with legacy fallback; UTF-8 content decoding via TextDecoder
• stringUtils.ts: New utf8ToBase64() and base64ToUtf8() utilities
• Translation files (10 locales): Add browseTab, changesTab, directory, emptyDirectory keys

Test plan

• Open Files tab in a git repo → "Changes" and "Browse" tabs visible
• Switch to Browse → directory listing loads, can navigate into subdirectories and back
• Open Files tab in a non-git workspace → auto-enters Browse mode, no error screen
• Tap a file with CJK/emoji characters in the path → opens correctly without crash
• View a file with Japanese/Chinese text content → displays correctly, no garbled characters
• View a binary file (image, etc.) → detected as binary, not rendered as text
• Return to Files tab from another screen → directory listing refreshes