feat(ui): file browser and connection string builder dialogs (#44)

Enter on File Path inputs opens a file browser (load/save, extension
filter, hidden toggle, overwrite confirm). Enter on Connection String
inputs opens a builder for PostgreSQL, MSSQL and SQLite with masked
password/preview, parsing and optional connection test.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

pkg/ui/ui_rules.md

@@ -205,12 +205,24 @@ Organize UI code into these files:
 - **column_screens.go** - Column editor, new column dialog
 - **domain_screens.go** - Domain list, domain editor, new/edit domain dialogs
 - **dialogs.go** - Confirmation dialogs (exit, delete)
+- **filebrowser_screens.go** - File browser dialog (`file-browser`), opened with Enter on File Path inputs
+- **connstring_screens.go** - Connection string builder dialog (`conn-builder`), opened with Enter on Connection String inputs
 
 ### Data Operations Files (Business Logic)
 
 - **schema_dataops.go** - Schema CRUD operations (Create, Read, Update, Delete)
 - **table_dataops.go** - Table CRUD operations
 - **column_dataops.go** - Column CRUD operations
+- **filebrowser.go** - Directory listing, extension filtering and start-path resolution (no tview)
+- **connstring.go**, **connstring_check.go** - Connection string build/parse/mask and connection test (no tview)
+
+### Input Dialogs
+
+- **File browser** - Enter on a File Path input opens it. Up/Down move, Enter opens a directory or selects a file,
+  Backspace/`[..]` goes to the parent, `h` toggles hidden files, `f` toggles the format extension filter,
+  `s` selects, Esc/`b` cancels (input unchanged). Save mode adds a file name field and asks before overwriting.
+- **Connection string builder** - Enter on a Connection String input opens it. Fields per type (PostgreSQL, MSSQL,
+  SQLite), masked password and preview, F2 save, F3 test connection, Esc cancels (input unchanged).
 
 ## Code Separation Rules