feat(tui): /rename, /cost, /continue <name>, ask_user picker, runtime…

[shenhao-stu]

Adds to frontends/tuiapp_v2.py:

• /rename : rename current session; collision-checked across in-memory sessions and the persistent registry. Topbar and sidebar refresh in place.
• /continue : resume by stored name in addition to the existing index form (/continue 1). The picker (/continue with no arg) labels each entry with its stored name when available.
• /cost [all]: per-session token usage (input / output / cache_create / cache_read), elapsed time, request count, and context-window %-left. The "all" form lists every tracked thread, including ended sessions.
• ask_user picker upgrades: free-text escape hatch on every list, Esc-back navigation, single/multi-select switch from a [多选]/[multi] question hint, candidate sanitizer that handles JSON-envelope debris.
• Runtime tip footer (└ Tip: ...) that rotates short usage hints.
• Topbar status chip showing the active model + idle/busy state.
• Cross-platform launcher hardening: auto-install of rich/textual on first run, and a hint when running under git-bash/mintty.

New modules:

• frontends/session_names.py: JSON sidecar at temp/model_responses/ session_names.json keyed by log-file basename; touched only by /rename and /continue . continue_cmd is not modified.
• frontends/cost_tracker.py: per-thread TokenStats, captured via monkey-patches on llmcore._record_usage and llmcore.print (Anthropic SSE's final output count only arrives via the latter). Idempotent install().

No core files touched.