feat: stacked mobile command menus and language-thread local-dev

[daopunk]

Summary

• Signal mobile menus: restack all command lists in menu_locale.rs (hub !help, transcription, !translation, privacy TEE cmds, unavailable) so each command sits on its own line with a short indented description — long !cmd — desc one-liners wrap badly on mobile Signal. Compound the layout into docs/solutions/signal-mobile-menus.md; module docs link to it. EN/ES stay structural twins; prose/status blocks stay paragraphs.
• Language Thread copy: localize sidecar title, description, and welcome via NEAR before create_group / invite (!translate-me-on); English target skips NEAR; failure falls back to English. Shorter main-group join confirmation ({lang} joined {name} thread).
• In-chat auto-translate detection: pair-constrained whatlang (Detector::with_allowlist) so short English (and other pair langs) are not mislabeled open-vocab (e.g. Norwegian); lower confidence bar inside the active pair.
• Docker proxy build: fix docker/Dockerfile.proxy for the current Cargo workspace (stubs for signal-bot / signal-bot-core / signal-bot-transcription; drop retired crates) plus Cargo registry retries / sparse index / longer HTTP timeout.
• Local-dev docs: add docs/local-dev/ for dual-stack bring-up, Signal captcha registration, verify SMS/voice, and log monitoring; link from root README.

Context

Long one-liner command menus break mid-phrase on mobile Signal (seen on !translation). Same pattern lived on hub help, transcription help, and privacy TEE lists — fix once and compound the standard.

Also unblocks local Docker bring-up (proxy image + captcha/registration guide) while we work toward a clean Phala CVM deploy, and improves Language Thread create/join UX plus short-message language detection for in-chat translate-all.

Test plan

Mobile menus

• cargo test -p signal-bot --lib commands::menu_locale
• cargo test -p signal-bot --lib commands::help commands::privacy
• Spot-check on mobile Signal: !help, !translation, !privacy (stacked cmds; no mid-phrase wrap on primary commands)

Language Threads / detection

• !translate-me-on es creates a sidecar whose name and welcome are Spanish (or English fallback if NEAR fails)
• Existing-sidecar join path (add_members) still works without re-creating the group
• Short English in an es/en auto-translate group routes correctly (not stuck as Norwegian / wrong side of the pair)

Docker / local-dev

• docker build -f docker/Dockerfile.proxy . succeeds
• Local compose/env using the proxy image starts without Dockerfile stub failures
• Follow docs/local-dev/README.md captcha → register → verify for one stack