refactor: split monolithic app.js into focused ES modules

[bin101]

Summary

• Replaces the 1450-line static/app.js with 12 focused ES modules (state.js, dom.js, api.js, format.js, schedule-matrix.js, athletes.js, config.js, settings.js, feed.js, status.js, tabs.js, main.js)
• Shared mutable state is isolated in state.js (single exported object) — no circular dependencies
• routes.py::serve_index generates an importmap that covers all local modules, so every module is cache-busted on each release
• Entry point changes from app.js → main.js; index.html updated accordingly
• Pure refactor — no behaviour change, all 380 tests pass, coverage 87 %

Module overview

Module	Responsibility
state.js	Shared mutable state (sport types, athlete caches, feed data, polling, run-button state, auto-save flag)
dom.js	$, toast, setButtonLoading, makeRemoveBtn, initRevealButtons
api.js	fetchJson, putJson
format.js	formatRelative, formatTime, formatSportLabel
schedule-matrix.js	7×24 schedule matrix: render, toggle, read, dim/enable
athletes.js	Athlete managed-list rows + search modal
config.js	Config tab: rules tables, loadConfig/saveConfig, auto-save
settings.js	Settings tab: loadSettings/saveSettings, quiet-hours, auto-save
feed.js	Feed tab: loadFeed, renderFeed, filter
status.js	pollStatus, pollLog, polling lifecycle, startRun, initRunButtons
tabs.js	activateTab, initTabs, initLangSelect
main.js	init() entry point

Test plan

• ruff check src tests — zero lint issues
• ruff format --check src tests — zero formatting issues
• mypy src — zero type errors
• pytest — 380 passed, coverage 87 %
• Manual: open the app, switch all tabs, save config, drag schedule matrix, search athletes, run dry-run

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