feat: multi-account, configure window, escalating notifications, polished installer

[psychofict]

Summary

Adds nine features to the widget while preserving single-account behaviour when only one account is configured.

• Multi-account — tray label aggregates across configured accounts (Work:67% Personal:12%); per-account Claude config dir; per-account Hide from tray and No poll flags.
• Configure window — tray menu → Configure… opens a tabbed Gtk window for editing accounts, thresholds, poll interval, and burn-rate alerts. Live-applies, no restart.
• Notification escalation — warn → critical → 100% tiers per window per account, persisted across restarts so a mid-window relaunch doesn't re-fire alerts. Re-arms when the window's resets_at shifts.
• Burn-rate alerts — configurable multiplier; 8-hour grace at the start of each new 7d window to avoid false alarms on low-elapsed-time data.
• Graceful failure UX — ! for auth/network/rate errors, ? for the brief gap between rollover and fresh data; last-known % preserved so transient blips don't blank the indicator.
• Compact reset times — countdown (2h 15m) when polling, absolute (9:00P / Th 7:00P) when polling is disabled for that account.
• Interactive installer — asks how many accounts to monitor, gathers label + claude_dir + creds.json check per account.
• pyenv detection — --copies venv that survives pyenv version switches; PyGObject pinned <3.51 on Ubuntu 22.04.
• Uninstaller — explicit prompt before removing user config.

Architecture changes

The single `claude_usage_widget.py` becomes four files:

• `claude_usage_widget.py` — indicator + poll loop only
• `shared.py` — config schema + utility helpers
• `config_window.py` — Configure UI
• `usage_popup.py` — details popup

Notable: v1.0.4's Preserve cached data during 429 rate limit overlaps the broader cache-preservation logic introduced here, so the older narrow form has been superseded.

Test plan

• Single-account install (default) launches and shows tray label as before.
• Multi-account: configure 2 accounts via Configure… → tray shows acc1:NN% acc2:NN%.
• Toggle Hide from tray on one account → it disappears from the label, stays in the popup.
• Toggle No poll on one account → reset time switches from countdown to absolute clock.
• Threshold notifications fire once each at warn/critical/100%; no re-fire after a process restart within the same window.
• Burn-rate alerts: enable in Configure, set multiplier to 1.0, confirm a notification once 8h have elapsed in the 7d window.
• Failure states: kill network → label shows !; restore → previous % returns; force a window rollover → ? shows briefly until next fetch.
• `install.sh` interactive flow asks for account count and per-account creds path.
• `install.sh` on a system with pyenv creates a `--copies` venv; switching pyenv version after install does not break the launcher.
• `uninstall.sh` prompts before removing `~/.config/claude-usage-widget/`.