refactor(ai): move chat + text-gen prompts into resources/ai/prompts/

Extracts the two inline LLM prompts (AiAssistantService's chat system
prompt and AiTextGenerationService's writing preamble) out of PHP
heredocs and into plain-markdown template files under resources/ai/
prompts/. Each file can now be edited without opening a service
class, without wrestling with PHP string interpolation, and with
proper markdown syntax highlighting in editors.

A tiny PromptLoader helper at app/Support/Ai/PromptLoader.php reads
the file and does {{placeholder}} substitution via strtr() — no
Blade, because Blade's {{ $var }} HTML-escapes ampersands and quotes,
which is wrong for LLM prompts (a company called "Smith & Co" would
be sent as "Smith & Co"). Missing templates throw RuntimeException
so they fail loud during development.

Pure refactor: no prompt wording changes. Existing AI feature tests
(AiChatFlowTest, AiGenerationTest) pass unchanged — they assert on
message structure via ScriptedAiDriver, not on prompt content. Three
new unit tests in PromptLoaderTest lock the helper's contract:
placeholder substitution, no-var loading, missing-file error.

app/Services/Ai/AiTextGenerationService.php

@@ -4,6 +4,7 @@ namespace App\Services\Ai;
 
 use App\Services\AiConfigurationService;
 use App\Support\Ai\AiException;
+use App\Support\Ai\PromptLoader;
 
 /**
  * Stateless one-shot text generation for the WYSIWYG popup.
@@ -67,13 +68,14 @@ class AiTextGenerationService
      * Keep the framing terse — text-generation output should not be padded
      * with "Here is the text you requested:" preambles. The instruction is
      * always placed last so the model gives it the most weight.
+     *
+     * The static preamble lives at resources/ai/prompts/text-generation.md.
+     * The conditional context + instruction appending stays here because it
+     * has different structure depending on whether `context` is present.
      */
     protected function buildPrompt(string $prompt, ?string $context): string
     {
-        $system = "You are a helpful writing assistant embedded in an invoicing application. Generate text based on the user's instruction. Rules:\n"
-            .'- Return only the requested text. No preamble, no explanation, no markdown code fences.'."\n"
-            .'- Match the tone and language implied by the instruction.'."\n"
-            .'- Be concise unless explicitly asked for length.';
+        $system = PromptLoader::load('text-generation');
 
         if ($context !== null && trim($context) !== '') {
             return $system."\n\n"