tech-debt: consolidate four near-duplicate time/size formatters

[Axelj00]

Problem

Four sibling formatters live in three different files, all with overlapping "compact unit-bucket" logic:

File	Function	Output
`src/tab-state.ts`	`formatElapsed(startMs)`	`0:45`, `1:15:30`, `7d 14h`
`src/tab-state.ts`	`formatResidentSize(bytes)`	`512K`, `156M`, `1.2G`
`src/notifications.ts`	`formatDuration(ms)`	`5s`, `2m 13s`, `1h 30m`
`src/notification-tray.ts`	`formatAge(ms)`	`5s ago`, `2m ago`, `3h ago`

`formatDuration` and `formatAge` are ~80% identical (same s/m/h/d bucketing, different suffix). `formatElapsed` is a third compact-time variant. The placement is locality-correct (each lives next to its caller), but the duplication will rot — a future tweak to the bucket boundaries needs to land in 2-3 places to stay consistent.

Fix

Consolidate into a single `src/format.ts` (or extend `tab-state.ts`) with:

```ts
export function formatDurationCompact(ms: number, opts?: { suffix?: string; precision?: "short" | "long" }): string;
export function formatElapsed(startMs: number): string; // keep (different shape — colons + days)
export function formatBytes(bytes: number): string; // rename formatResidentSize
```

Then `formatAge(ms)` becomes `formatDurationCompact(ms, { suffix: " ago" })` and `formatDuration(ms)` becomes `formatDurationCompact(ms)` with the existing call sites updated.

Out of scope

• Localization ("5s" → "5 seconds"). Current consumers all use the short form intentionally for footer / tray density.
• Pluralization ("1m" vs "2m" → no plural "s"). Already by design — these are compact, not English.

Files

• `src/tab-state.ts` (formatElapsed:344, formatResidentSize)
• `src/notifications.ts` (formatDuration)
• `src/notification-tray.ts` (formatAge)