feat: remember last-used charset / color in localStorage

[can4hou6joeng4]

Why

First-time visitors discover a charset they like (often braille or blocks), leave, and come back to the default. A tiny localStorage restore would make the tool feel sticky without accounts.

Scope (keep small)

• Persist charset and color mode only (not brightness/contrast — those are image-specific).
• Key namespaced, e.g. semaphore-charset, semaphore-color.
• Read on boot in src/tool.ts after the control DOM is wired; write on change.
• Ignore malformed values; fall back to current defaults.
• Theme already uses localStorage (semaphore-theme in shared.ts) — mirror that style.

Out of scope

• Syncing across devices
• Persisting the last image (privacy + storage)
• Server-side anything

Acceptance

• Reload /tool keeps the last charset and color
• Clearing site data restores defaults
• npm test and npm run build still pass
• No new network calls; still fine under connect-src 'none'

Pointers

• src/tool.ts — control wiring
• src/shared.ts — existing theme localStorage pattern
• STYLEGUIDE.md — copy voice if you touch any user-visible string