Bug-fix and polish release.

Fixes
- Emoji no longer disappear from processed sentences. Sites like YouTube load
comment emoji lazily; if Seep processed the text before the emoji image had
loaded, it showed as a blank space, both in the translated sentence and when
toggling back to the original English. Seep now shows the native emoji
character instead.
- The underline beneath the selected word now scales with the text size. On
large text it previously floated visibly above the sentence highlight, and
its placement on Firefox differed slightly from Chrome. It now sits
correctly at every font size, and normal-size pages look exactly as before.
- Opening a sentence while the sidebar was collapsed to its edge tab now
selects a word right away, so the keyboard rating shortcuts work
immediately. Previously nothing was selected until you expanded the sidebar.
- Uninstalling while signed out now opens the goodbye page, and feedback there
can be sent without an account.

Improvements
- The collapse-sidebar shortcut now has a default key: C. Custom key bindings
you already saved are kept.
- Expanding the sidebar from its collapsed tab no longer replays the slide-in
animation; the tab grows into the panel in place.
- Sentence audio is hidden during the onboarding tutorial and appears once
normal browsing begins.