fix: web theme toggle shows light/dark only, never the system icon

[time-attack]

Problem

The web UI's theme toggle (<theme-toggle> from @mariozechner/mini-lit) would sometimes render the computer/monitor (system) icon instead of sun/moon.

Root cause is in the third-party component:

• It treats absence of a localStorage.theme key as "system".
• getIcon() returns the Monitor icon whenever theme === "system" — regardless of the includeSystem prop.

So any user who never set a theme (fresh load), or whose 3-state cycle landed on system, got the computer icon. We can't edit the compiled dependency, so this is fixed at our only consumer, plugins/web-ui/src/shell.ts.

Fix

• .includeSystem=${false} — the toggle cycles light ↔ dark only and can never land on system.
• On load, seed a concrete light/dark into localStorage.theme from the OS prefers-color-scheme when none is set, so the element never initializes to "system" and never paints the Monitor icon.

Behavior

First load reflects the user's OS preference (light or dark); after that the toggle just flips light↔dark. This intentionally drops OS-follow behavior in favor of an explicit two-state toggle — the reported bug was the system/computer state being unwanted.

Verification

• tsc --noEmit clean, prettier + oxlint clean on the changed file.

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