Minor bug fixes and code cleanup (B15, B16, Phase 6)

[Ashex]

Context

Final cleanup items from the v2 rewrite analysis that don't warrant individual issues.

Sub-tasks

Minor bugs

• B15: config_set_str silently drops entries beyond CONFIG_MAX_ENTRIES (bumped to 128 but still silently drops). Either make the list dynamic or log a warning when the limit is hit.
• B16: Disk cache filenames still use str_hash() without storing the full URL for collision verification. In-memory cache is already collision-safe. Low priority — store URL alongside hash in disk cache metadata, or switch to a longer hash.

Code cleanup (Phase 6)

• Remove stale legacy v1 source files if any remain (old compose_view, login_view under src/ui/)
• Audit and remove stale comments referencing removed features (compose, reply, old login flow)
• Review grep -rn 'TODO\|FIXME\|HACK' src/ output and resolve or document remaining markers
• Remove the "Nickel Plugin Port" section from README since plugin-port.md is being deleted
• Clean up README references to docs/ directory after docs deletion