fix(reader): flush location on close

[codedogQBY]

Analysis

• [Bug] 不能记录上次阅读时的位置，而是会回到章节开始 #194 reports that reopening a book can return to the beginning of the chapter instead of the last read position.
• Desktop ReaderView saved relocate progress through a 5s throttled updater. If the user closed the reader/app before the trailing throttled write fired, the latest CFI never reached the library store/database.
• Mobile already has an immediate unmount save path; desktop did not.

Changes

• Track the latest desktop relocate progress/CFI in a ref before the throttled save runs.
• Flush that latest position on component unmount, pagehide, and beforeunload.
• Keep the existing throttled save for normal reading so frequent relocate events do not spam writes.
• Reset the latest-progress ref when switching books/tabs/formats.

Verification

• pnpm --filter app exec tsc --noEmit
• git diff --check

Fixes #194