mobile: fix push notif rendering

[latter-bolden]

The problem here was #5839 adding a new markdown parsing dependency that crashes when run from the scripts bundle — our minimalist JS target that gets run natively on iOS via JavaScriptCore.

The runtime path we use from scripts is getTextContent. That helper is contained in a module that requires extractTables, which expects the DOM to be available at runtime and crashes if it's not.

I think 930b2fa alone fixes the immediate problem by changing the esbuild conditions to target RN. However, respecting that is opt-in at the module level so simply configuring the build command doesn't prevent similar issues in the future.

The other commits add a new postNotificationText helper that's intentionally limited in what it imports and is placed within the scripts module itself. I don't love having another "turn raw message into text string" helper, but I think the alternative is a thorny disentangle of postContent.ts that might regress down the line.

Implementation wise, the new helper produced identical output against post fixtures and has seemed fine anecdotally in the Preview App. However, if we're concerned about the added helper, we can leave it out and exclusively merge the esbuild fix.

Fixes TLON-5843

[claude]

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[linear]

TLON-5843

[jamesacklin]

@claude review once

[dnbrwstr]

Lgtm