fix: prevent mobile Safari from caching API responses

[btucker]

Summary

Mobile Safari PWA aggressively caches fetch responses, which can cause stale channel history when returning from background. Messages posted by channel leads while the app was backgrounded would not appear on mobile but were visible on desktop.

• Backend: Added Cache-Control: no-store header to all API responses via tower_http::SetResponseHeaderLayer
• Frontend: Added cache: "no-store" to fetchHistory and fetchAgentsMd fetch options as defense-in-depth

Test plan

• cargo clippy clean
• cargo test — 1534 lib tests pass
• biome check — 0 errors
• Manual: open mobile PWA, background it, post a message from desktop, foreground mobile — message should appear

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.92%. Comparing base (b47707c) to head (27fa5d3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2329      +/-   ##
==========================================
- Coverage   63.93%   63.92%   -0.02%     
==========================================
  Files         100      100              
  Lines       38578    38581       +3     
==========================================
- Hits        24664    24661       -3     
- Misses      13914    13920       +6     

Files with missing lines	Coverage Δ
src/daemon_v2/web/mod.rs	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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