feat: mobile UX improvements and session validation

[TeigenZhang]

Summary

• Mobile logo: Show Codeman logo on mobile as a compact home button (previously hidden on small screens)
• Expandable history: Display initial 4 history sessions with a "Show More" button to expand all, reducing visual clutter
• Session deduplication fix: Deduplicate by projectKey instead of workingDir to fix lossy decode issues
• Project key decoding: Handle _ encoded as - with look-ahead logic for correct path reconstruction
• Resume session validation: Pre-validate resumeSessionId before passing to Claude CLI to prevent invalid session errors
• Content validation: Apply content validation to all session files regardless of size

Changed files

• src/web/public/mobile.css — Mobile logo styling
• src/web/public/styles.css — History section expand/collapse styles
• src/web/public/terminal-ui.js — History rendering with dedup fix and expandable UI
• src/web/routes/session-routes.ts — Session validation and project key decoding fixes

Test plan

• Verify mobile logo renders correctly on iPhone/Android viewports
• Verify history shows max 4 items initially, "Show More" expands all
• Verify sessions with similar working dirs are properly deduplicated
• Verify resume with invalid session ID is handled gracefully
• Verify session file content validation works for all file sizes