feat: incremental focus ladder + a deeper Better Stats window

[gluansinha-star]

What this adds

Two opt-in features on top of upstream Pomotroid 1.7.1. Both are off by default, so the app behaves
exactly like upstream until you turn them on.

1. Incremental Focus

Each completed focus round makes the next one longer, up to a ceiling you choose.

Configured in Settings → Timer → Incremental Focus:

Setting	Meaning	Default
Incremental Focus	Master on/off switch	off
Add per Round	Minutes added after each completed focus round	5 min
Maximum Focus	Ceiling for the escalated duration	90 min

With a 25-minute focus and a 5-minute increment capped at 45 minutes the ladder runs
25 → 30 → 35 → 40 → 45 → 45 → …, and the settings panel renders a live preview of it.

Behaviour:

• The first round uses the configured Focus duration as-is; the increment starts from round two.
• The ladder resets after a long break and on Reset, so every cycle starts from the base.
• Breaks are never escalated — only work rounds grow.
• A cap below the base duration is clamped up, so a round can never get shorter than the configured
  Focus time.
• While a work round runs, a caption under the round label shows the position, e.g. 25m +10m · step 3,
  plus · capped once the ceiling is reached.

The ladder lives in SequenceState and reaches the frontend through six new TimerSnapshot fields.
Three new settings persist via MIGRATION_7, so existing databases upgrade without data loss.

2. Better Stats

A new window opened from the titlebar, sitting next to the original Statistics window (which is
untouched and still works exactly as before).

• Momentum — today vs your trailing 7-day average as a progress ring with a plain-language
  verdict, a streak strip that warns when a live streak has nothing logged today, week-over-week
  comparisons for rounds / focus time / active days / average per active day, and rolling 7, 28 and
  90-day windows.
• Progress over time — a 28-day area chart with the 7-day moving average overlaid, and a
  day/week toggle; 12-week focus-minutes and rounds bar charts.
• Consistency — an interactive heatmap with Recent (120 rolling days), Month (a real
  calendar with day numbers, today outlined, future days dashed and ‹ › navigation) and Year
  scopes; a habit-strength donut splitting the last 30 days into strong / light / rest; steadiness
  (standard deviation of daily rounds), best run, your focus window, and a round-length histogram.
• Rhythm — a weekday × hour grid showing when you actually focus with the peak slot called out,
  plus ranked focus-by-weekday and focus-by-hour breakdowns.
• Personal bests — longest round, best day by rounds and by focus time, best week, average round
  length, tracked days, streaks.

Everything arrives in a single IPC call (stats_get_insights) so there is no staggered loading, and
charts are hand-rolled SVG with no new dependencies. Durations read naturally: 45m, then 1h 30m,
and whole hours past ten (12h).

Bug fix

While building the stats window I hit a real bug that made several panels render as zeros:

• day_num_to_date() was shifted by one day, so every date it produced missed every real session.
• weekday_index() was misaligned by one, skewing the weekday profile and the weekday-by-hour grid.

Together these made fill_recent_days() (the 28-day trend) and roll_up_weeks() (the weekly chart)
build windows containing dates that matched no history. Both are corrected and pinned by tests,
including exact day numbers for known dates and round-trips across leap years, 1970-01-01 and
2100-03-01.

Screenshots

Incremental Focus	Better Stats — momentum

| Better Stats — consistency | Better Stats — rhythm |

Testing

• npm run check — 0 errors, 0 warnings.
• npm run tauri build — produces the NSIS installer and MSI successfully.
• Manually verified in a running build: the ladder steps correctly and caps, resets after a long
  break and on Reset; the stats window renders all sections and the heatmap navigates across months
  and years.

Not yet run: cargo test. Tests are written for the new ladder logic, the date fix and the new
query helpers, but the suite has not been executed. Worth running before merge.

Notes for reviewers

• src-tauri/src/db/queries.rs carries most of the new Rust; the insights layer pulls the full
  history once and derives every metric in Rust, which keeps the SQL simple and the snapshot
  consistent.
• src/lib/dev/mockTauri.js is a dev-only IPC stand-in so the UI can be previewed under plain
  npm run dev in a browser. It is guarded by import.meta.env.DEV and never enters a production
  build, but it is the one addition worth a deliberate look if you'd rather not carry it.
• The three new message keys per feature were added to all 8 locales with English fallbacks via
  scripts/sync-messages.mjs; real translations are still needed for the non-English locales.