feat(seer): track sidebar resize and position analytics (#123058)

Adds Amplitude events so we can measure Seer sidebar size preferences.

- `seer.explorer.sidebar.resized` on committed split resize
(`onResizeEnd`): `seer_size`, `seer_size_percent`, `orientation`,
`browser_width`, `browser_height`
- `seer.explorer.sidebar.position_changed` when dock position changes:
`position`, `browser_width`, `browser_height`

Drawer-mode resize is unchanged (still no event); this covers the
persistent sidebar path.

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Requested by **nate**.
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Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>
Co-authored-by: Nate Moore <nate.moore@sentry.io>
Co-authored-by: Nate Moore <natemoo-re@users.noreply.github.com>