Precise risk-free entry, and autoload the latest saved run

[GusFurtado]

Context

Two more items from feedback. Stacked on #22.

Risk-free rate precision

• Slider step tightened from 5 to 1 (81 positions instead of 17 — 5x finer dragging resolution).
• The read-only value display is now an editable text field: type an exact rate directly rather than fighting the slider for it.
• Locale bug avoided, not just noticed: I initially built this as <input type="number">, but a number input's displayed decimal separator follows the browser's locale (this session's browser renders commas) while its underlying .value is always period-formatted regardless — a recipe for someone typing what looks like a completely normal number in their own locale and having it silently fail to parse. Switched to plain text with inputmode="decimal", and comma is accepted as an alias for the decimal point when parsing.
• Commits on change (blur/Enter), not input, and the render loop skips reformatting the field while it's focused, so typing doesn't fight itself and an unrelated render() firing mid-edit (e.g. the running overlay finishing) doesn't reset the cursor.
• Real, previously-unnoticed bug fixed along the way: nothing ever moved the slider's thumb except its own drag handler — toggling units, typing an exact value, or loading a saved run with a different rate all left the thumb wherever it last was, out of sync with the actual state. renderRfControl() now syncs it on every render.

Autoload the latest saved run

init() now loads the most recent saved run automatically (saved runs are stored newest-first) if one exists, instead of always starting from the empty state. First-time visitors with nothing saved still get the get-started card.

Verification

Headless Chromium: typing an exact rate (including comma-decimal) moves both the display and the slider correctly and round-trips through the annualized toggle; a genuinely-focused mid-edit survives an unrelated render() unreformatted; invalid input reverts cleanly; loading two seeded saved runs on a fresh page auto-selects the newest one and fires a real analyze call with its tickers/period.

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