[PurpleRain-PR]

What this PR adds

1. Daily pseudo-deadline window (opt-in)

A new settings page (Settings → DDL 每日伪截止), default off with a preset 22:00–08:00 window. Start/end times are picked with the native Material time picker (dial, with keyboard toggle); a separate minutes-precision text input is not needed and not offered.

• A deadline due inside the window is displayed and laid out at the window start as its effective deadline. The real due time is never rewritten: the projection is presentation-only and lives in app settings, so toggling the window on/off or moving its hours can never overwrite user data.
• Handles overnight windows (early-morning deadlines group to the previous night's start), inclusive endpoints, and a start equal to end — which then matches only that single instant, at minute precision.
• On the timeline, an adjusted deadline draws a wide slanted band between the effective and real deadlines that scales with zoom, with the real marker faded and always painted last (even under overlap). Adjusted cards carry a compact label, and list rows show the original deadline underneath.

2. Timeline layout width — global default with per-item tails

• A global layout width D (default 4 days, range 1–30) controls how far DDL cards and the event/exam info area reserve space to the left.
• Per deadline, the editor's "更多选项" offers a 1–30-day tail slider plus a "跟随全局设置" switch. The choice is stored per entry as an optional tailDays — null/missing means follow the global setting, so existing files keep loading unchanged.
• A developer-only migration button ("将默认拖尾长度设为 null", under 关于弹窗五次连击解锁的开发者选项) converts stored 4-day tails — including recycled entries — to follow-global in one write, leaving custom lengths alone. Nothing migrates automatically.

3. Event and exam timeline cards

• The real span of an event or exam is painted as 45° two-tone wide stripes between the start and end markers, scaled to the actual duration.
• The left info area (title, times, course, location) uses the matching hatch with an inward accent border, so a card reads as one continuous object.
• A new 日程与考试信息栏 setting (default on) switches these cards to the old plain box style — title and time inside the box, left reservation released — without touching DDL or open-ended entries.

4. Consistency groundwork

One shared caption helper now renders the time line for every card style (deadline, event, exam), and the timeline grid header reuses the same short-day formatter; the 1–30 day tail range is sourced from a single model constant used by the editor, the settings page and layout validation alike.

Data & compatibility

• New optional tailDays key in the agenda JSON; absent means inherit. No storage schema change and no load-time writes.
• The cutoff window is saved as app settings only; enabling, re-timing or disabling it never writes agenda files.

Docs

New docs/deadline-cutoff.md (time rules, layout reservation, card styling); docs/architecture.md updated.

Testing

GitHub Actions on the PR head is the arbiter for all builds and the full widget/unit suite.