[WayneKent]

Problem

The preview is capped at 8 KiB, but the cap was applied with a plain byte cut
(w.buf.Truncate(maxPreviewLength)). When that cut lands inside a multi-byte
rune, the preview ends in a partial UTF-8 sequence.

DuckDB rejects an invalid UTF-8 string bound as a VARCHAR, so

INSERT INTO events (primary_mime_type, time, preview) VALUES (?, ?, ?)

fails and the daemon only reports:

ERRO failed to insert data
  error=
  │ could not bind parameter

The transaction rolls back and the whole clipboard event is lost: yankd list
never shows it.

Because CJK runes are 3 bytes, 8192 % 3 == 2, so for Chinese content the cut
almost always splits a rune. ASCII copies are unaffected — that is why only some
copies were lost. Reported case: copying a long Chinese markdown document from a
web page.

Change

• Add runeSafeLength(buf, n), which backs the cut off to the start of the last
  partial rune, so the preview is always valid UTF-8.
• At most 3 bytes are dropped: the buffer only ever holds valid UTF-8, because
  only entries whose IsText is true are appended to it.
• The preview is still capped at 8 KiB by bytes; only the cut point moved.

Tests

TestGeneratePreivewKeepsPreviewValidUTF8 covers:

• 3-byte runes, where the cap lands mid-rune
• 4-byte runes, where the cap lands mid-rune
• 4-byte runes with an aligned cap, and pure ASCII — these two also fail if the
  fix ever over-trims by a fixed offset

The first two fail without the fix and pass with it.

Reproducing on the unfixed version

Only payloads larger than 8 KiB are affected, so any clipboard content above
that size with multi-byte characters reproduces it:

# 1. build a payload that is just over the cap
line='这是一段用于复现预览截断问题的中文内容，复制它会触发 yankd 的预览生成逻辑。'
yes "$line" | head -n 75 > /tmp/yankd-repro.md   # 8250 bytes

# 2. sanity check: the 8 KiB cap really does land inside a rune
head -c 8192 /tmp/yankd-repro.md | iconv -f UTF-8 -t UTF-8 >/dev/null \
  && echo "aligned — shift the payload by one ASCII byte and try again" \
  || echo "this payload hits it"

# 3. put it on the clipboard (needs wl-clipboard; copying the same
#    8250-byte text from any app works too)
wl-copy --type text/plain < /tmp/yankd-repro.md

With the unfixed build and the daemon running:

• the daemon logs ERRO failed to insert data ... could not bind parameter
• the entry never appears in yankd list

A browser copy of the same text also hits it — a browser additionally advertises
escaped \uXXXX variants of the text, which only moves the cut point, so if a
particular document happens to be aligned, add or remove one ASCII character and
copy again.

[Nadim147c]

Thanks for the pull request!