ui: detect the conversation import format from file contents (#26121)

* ui: detect the conversation import format from file contents

iOS resolves every accept entry to a UTI and has none for ".jsonl", so the
picker greyed out exported conversations. Drop the accept filter and pick the
parser from the file contents: ZIP magic bytes, then a first "session" record
for JSONL, otherwise the legacy JSON format.

Also remove the unused importConversations() picker and an orphan doc comment,
and cover each format with unit tests.

* ui: report what a conversation import actually wrote

The import summary echoed the selection back, so re-importing conversations
already in the database claimed success while nothing was written and only a
console warning said otherwise.

Return the imported and skipped conversations from the database layer, list the
written ones in the summary, and count the rest in a toast.

* ui: name the literals of the JSONL conversation format

Introduce SessionRecordType and SESSION_HARNESS, and reuse the existing
NEWLINE constant, so the record format lives in one place.

This also covers the writer side, which predates the import path under
review and carried the same literals: an enum stated by the reader alone
lets the two sides drift.

Values are unchanged, so an export stays byte identical.

tools/ui/src/lib/constants/conversation-import.ts

@@ -0,0 +1,3 @@
+// First bytes of every ZIP local file header ("PK"). Import detects an archive
+// from these bytes rather than from the filename, which the OS may not preserve.
+export const ZIP_MAGIC = [0x50, 0x4b];

tools/ui/src/lib/constants/index.ts

@@ -13,6 +13,7 @@ export * from './storage';
 export * from './attachment-menu';
 export * from './auto-scroll';
 export * from './context-gauge-popup';
+export * from './conversation-import';
 export * from './binary-detection';
 export * from './built-in-tools';
 export * from './cache';

tools/ui/src/lib/constants/message-export.ts

@@ -7,6 +7,9 @@ export const EXPORT_CONV_NAME_SUFFIX_MAX_LENGTH = 20;
 // Characters to keep in the ISO timestamp. 19 keeps 2026-01-01T00:00:00
 export const ISO_TIMESTAMP_SLICE_LENGTH = 19;
 
+// Producer marker carried by the session record of a JSONL export
+export const SESSION_HARNESS = 'llama.app';
+
 // Replacements for making the conversation title filename-friendly
 export const NON_ALPHANUMERIC_REGEX = /[^a-z0-9]/gi;
 export const EXPORT_CONV_NONALNUM_REPLACEMENT = '_';